As a methodology, unit testing has many advantages over manual, ad-hoc testing and debugging. By developing database unit tests, you can create a collection of tests and run them during development to ensure that your features work as you expect. Because each unit test focuses specifically on an individual method, you can more easily determine the source of a failure for a failing unit test. Therefore, database unit tests help you determine the sources of bugs in your code.

Such a collection of tests is very useful for regression testing. As you implement new features, you can rerun existing tests to ensure that existing functionality has not been broken. Such a regression test suite facilitates database changes, because you can now make changes knowing the implications of those changes.

Unit tests, in addition, serve as documentation for users of the methods under test. Developers can quickly review unit tests to determine exactly how particular components should be consumed.

I agree that regression testing is very useful. With that being said, keeping the regression scripts up to date is crucial in order to keep them relevant and meaningful. Otherwise, the likelihood of finding defects will diminish. Ideally a periodic review of regression scripts would be built in to the overall process.